The detection of low-energy particles is a task relevant to many fields. Such fields non-exclusively include electron microscopy, astronomy and electron beam lithography. In certain applications, it may also be desirable for surface analysis devices to utilize low-energy charged particle detection. Additionally, there is often a need for low-energy charged particle detectors in scientific experiments and applications.
Various types of detectors have been used for these applications. They include scintillation-based detectors such as Everhart-Thornley detectors that convert low-energy charged particles into photons and then convert the photons into an electrical signal; solid-state devices akin to photodiodes or phototransistors (cathododiodes or cathodotransistors); cathodconductivity devices, and MSM devices (described in an article by G. D. Meier et al, J. Vac. Sci. Tech. B14, 3821 (1996)).
Cathododiodes may be made in the form of pn-junctions, pin-junctions, avalanche diodes or Schottky barriers. Typically, when a charged particle such as an electron is incident upon a semiconductor layer in the cathododiode, it creates electron-hole pairs.
The functional properties of a semiconductor result, in part, from providing electrons in different energy states separated by bands or gaps of no energy states. The highest occupied band is a valence band and the lowest unoccupied band is a conduction band, with a gap existing in between. As used, the terms “highest” and “lowest” refer to energy levels and not physical vertical separation. When an electron or other ionizing radiation strikes a semiconductor detector, it will excite electrons present in the valence band of the detector into the conduction band, consequently leaving holes in the valence band. This process is known as the creation of electron-hole (“EH”) pairs.
The creation of an electron-hole pair provides two charge carriers that are opposite in polarity (an electron and a hole). With respect to these carriers, the non-dominant carrier is typically referred to as the minority carrier while the dominant carrier is referred to as the majority carrier. The roll of an electron as a minority or majority carrier is determined by the device configuration. Solid-state detectors typically provide an electrical field via a depletion region (a built in field) and/or an applied potential as a means for separating these carriers. It is understood and appreciated that certain types of devices, such as cathodoconductivity devices do not provide a depletion region.
Charged particles created in such an electric field (built in or applied) will tend to be swept out of it. For example, in a pn-diode after an EH pair is created a positive charge carrier will be swept towards the p-type region by the depletion layer's electric field, and a negative charge carrier will be swept towards the n-type region by the depletion layer's electric field.
In a diode, including a cathododiode, the movement of these charge carriers constitutes a current that can be measured. For the current induced by the generation of EH pairs to be measured, the resulting charge carriers must survive for a duration of time sufficient to permit them to be swept across the depletion region.
The penetration depth of low-energy particles incident upon a semiconductor is quite short. For example, the penetration depth, or Grun range, of electrons with less than 1 keV of energy is less than 10 nm in most semiconductors. At 100 eV the penetration depth is typically only a few nanometers, or less. As such, the EH pairs that are created are created very close to the surface of the semiconductor.
Conventional semiconductor fabrication processes typically generate defects such as dangling or frustrated bonds at the surface. These and other surface defects, (such as, for example oxidation) cause problems such as surface recombination, surface band-bending, surface traps and other surface related conditions that can thwart the detection of the EH pair, by causing charge carriers created close to the surface to recombine before they are swept across the depletion region.
Cathodotransistors also rely on the creation of EH pairs and the consequent changes in carrier densities. These changes in carrier density affect the height of the energy barriers between layers of the device that gate a flow of carriers across the layers. Like the cathododiodes described above, the performance of the cathodotransistors is adversely impacted when the generated carriers (the EH pairs) are generated in close proximity to a surface that causes most of the carriers to recombine quickly. Thus, cathodotransistors can also have a low efficiency in the detection of low-energy charged particles.
Similarly, the efficiency of cathodoconductivity-based devices can be adversely impacted by semiconductor surfaces that reduce the lifetime of generated carriers. Scintillating materials also tend to radiate less efficiently when stimulated by low-energy charged particles due to common occurrence of surface defects.
In addition, many of the above-described devices are susceptible to having large dark, or leakage, currents that make it difficult to detect the signal currents generated by the low-energy particles.
Hence, there is a need for a low-energy particle detector semiconductor device that overcomes one or more of the drawbacks identified above.